Aliphatic polyamides, particularly nylon 6 and nylon 66, are extensively used in a variety of industrial and consumer products such as carpets and automotive parts. In particular, carpets and automobile air bags contain large portions of polymers with a high polyamide content. Because of the great quantity of post-industrial and post-consumer nylon made available each year, these nylon products are ideal for recovery and recycle. Additionally, concerns over efficient resource utilization and environmental protection have created a need for the recovery and recycle of nylon from discarded post-industrial and post-consumer products.
Recycle processes are already used to recycle polyamide carpet waste in order to minimize the portion of the polyamide-containing carpet waste that has to be discarded. Mechanical means, such as grinding and crushing, are known means for separation of solid polyamide material from foreign materials such as carpet backing, etc. Mechanical separation yields a low grade recycled product with limited uses. In order to produce a high-quality recycled polyamide product, the process must remove impurities such as dyes, cotton thread, delusterants (TiO2), dirt, and oil, among other things, that cannot be removed by mechanical means alone.
There are several available non-mechanical approaches to reclamation of polyamide from discarded polyamide-containing products by isolating the polymer. Polyamides such as nylon are soluble in selected solvents, and thus solution-based processes offer routes to the recycle and recovery of polyamides. Suitable solvents are polar and often reactive with the nylon. Many need to be handled with extreme caution for safety reasons. From a processing point of view, ideal solvents should have the following characteristics: environmental friendliness, cost-effectiveness, low toxicity, capability of dissolving polyamides at relatively low temperatures, and capability of inducing polyamide precipitation for subsequent separation from the solvent. As an additional consideration in solvent-based recycle and recovery of nylon, a single solvent system rather than a mixture or a solution is generally desired as a cost effective and easier to operate system. Certain polyols and carboxylic acids have many attributes of ideal solvents for polyamide recycle and recovery. However, polyols and carboxylic acids have not been attractive solvents because they are reactive with polyamides, and thereby have contributed to the degradation of molecular weight of the polyamide. In these prior art processes, slight losses in molecular weight have been tolerated, however, it has been thought that more significant degradation is to be avoided because recovered degraded polyamides are unsuitable for either extrusion purposes (e.g. fibers and films) or use as molding compounds.
U.S. Pat. No. 5,430,068 to Subramanian discloses a process to recover polyamides using anhydrous polyols or aliphatic carboxylic acids having from 2 to 6 carbon atoms as the solvent. The process also includes the step of rapidly quenching the polyamide solution with an additional quantity of solvent to avoid any significant degradation of the polyamide.
Other known solvent-based recycle and recovery processes use glycols as solvents, such as acetic acid and water, ethylene glycol and propylene glycol. Glycol-based solvent processes take advantage of the different solvencies of nylon 6 and nylon 66 at particular temperatures to separate one from the other. However, glycols also react with the polyamides, in this case to create higher molecular weight polyamides. Thus, the residence time, i.e. the time that the polyamide is contacted with the solvent must be short to avoid glycol reaction with the polyamide.
Aliphatic alcohols have been suggested for use as solvents in processes to recycle and recover polyamides. Methanol, in particular, has been shown to be useful in the separation of nylon 6 from nylon 66. Moreover, aliphatic alcohol solvents are effective under mild conditions, i.e. low temperature and short residence time.
U.S. Pat. No. 5,840,773 to Booij et al discloses a process to recover polyamides from carpet waste using an aliphatic alcohol as the solvent. The process conditions are such that “virtually no polyamide is decomposed” so that directly reusable polyamides are obtained from the process.
U.S. Pat. No. 6,036,726 to Yang et al also discloses a solvent-based process to recover polyamides. Under the conditions described, the molecular weight of the recovered polyamide is “substantially unchanged”, making the polyamide readily available for re-use.
In these prior art processes, therefore, degradation and depolymerization of the polyamide is avoided so that the recovered polyamide can be readily and directly reused to make finished products. One drawback, however, is that the viscosity of the unchanged polyamide solution remains relatively high, making filtration with fine filters difficult. Smaller, finer particles, such as sub-micron TiO2 particles, are difficult, if not impossible, to separate from the polyamide solution. Thus, because the polyamide itself remains non-degraded in the existing recycle and recovery processes, it is difficult to mechanically or chemically remove smaller impurities such as TiO2, a delusterant, that may be part of post-industrial and post-consumer nylon products.
This problem is addressed by the present invention in which a solvent-based process is provided to recover polyamides from post-industrial and post-consumer products. The process of the present invention allows for the separation of TiO2 and other fine insoluble particles by partially depolymerizing the polyamide to decrease its viscosity.